This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to a more efficient method of data encoding in dynamic resonance imaging through the transfer of k-space data to the time domain thereby acquiring a more dense k-t space with attendant reduction in acquisition time.
Magnetic resonance imaging (MRI) is a nondestructive method for the analysis of materials and represents a new approach to medical imaging. It is generally noninvasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited using magnetic fields which rotate at specific frequencies proportional to the local static magnetic field. The radio frequency signals resulting from the precession of excited spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin distribution of the body.
FIG. 15A is a perspective view, partially in section, illustrating coil apparatus in MR imaging system and FIGS. 15B-15D illustrate field gradients which can be produced in the apparatus of FIG. 15A. Briefly, the uniform static field B.sub.0 is generated by the magnet comprising the coil pair 10. A gradient field G(x) is generated by a complex gradient coil set which can be wound on the cylinder 12. An RF field B.sub.1 is generated by a saddle coil 14. A patient undergoing imaging would be positioned along the Z axis within in the saddle coil. In FIG. 15B an X gradient field is shown which is parallel to the static field B.sub.0 and varies linearly with distance along the X axis but does not vary with distance along the Y or Z axes. FIGS. 15C and 15D are similar representations of the Y gradient and Z gradient fields, respectively.
FIG. 16 is a functional block diagram of conventional imaging apparatus. A computer 20 was programmed to control the operation of the MRI apparatus and process FID signals detected therefrom. The gradient field is energized by a gradient amplifier 22 and the RF coils for impressing an RF magnetic moment at the Larmor frequency is controlled by the transmitter 24 and the RF coils 26. After the selected nuclei have been flipped, the RF coils 26 are employed to detect the FID signal which is passed through the receiver 28 and then through digitizer 30 for processing by computer 20. For the dynamic imaging techniques of the present invention, physiological monitoring and triggering equipment (not shown) may be needed, as known by one skilled in the art.
The signal collected in magnetic resonance imaging (MRI) is a sample of the Fourier transform of the object being imaged. MRI is used in several applications to monitor the time behavior of an organ of interest. In such dynamic applications the signal collected belongs to a Fourier space augmented with a time axis, known as a k-t space. When imaging a dynamic object, an ideal situation would consist in completely filling the corresponding k-t space matrix, thereby providing all the desired spatial information at any moment in the imaging period. However, the imaging process is often too slow to acquire all the spatial and temporal information with the needed resolution. Various methods have been developed to increase the resolution of an acquired data set. The simplest approach is to interpolate the data spatially (e.g. zero filling) or temporally (e.g. data sharing), resulting in a smoothed representation of the actual object. A second approach consists in using a limited number of high resolution images to model the spatial variations of the imaged object (e.g. keyhole, `reduced-encoding MR imaging with generalized-series reconstruction`, `singular value decomposition`). Time frames can be acquired in rapid succession (high temporal resolution) as the model requires only a small amount of spatial information to generate a full FOV time frame. However, only the dynamic changes having a spatial distribution expected by the model are depicted accurately in the resulting images. Another approach consists in assuming the dynamic changes to be contained within only part of the FOV. The prior knowledge that some regions of the object are static is used to reduce the amount of information that needs to be acquired.
Unlike the methods above, the present invention does not rely on interpolation, modeling or assuming part of the FOV to be static in order to increase resolution. Instead, the invention provides new flexibility in the way spatio-temporal information is encoded with MRI, such that one can avoid some inefficiencies present in conventional acquisitions.